Reconstructed cell-free liver systems and isolated rat hepatocytes are used in studies: (1) of the energetics of transmembrane movements of hydrogen equivalents and of metabolites with respect to intracellular control of metabolism and the unique effects of ethanol on liver metabolism; and (2) of factors which regulate the relative rates of carboxylation and decarboxylation of pyruvic acid as initiation points of metabolism for major metabolic processes. Surgically isolated perfused rat hindquarters (muscle) are used in studies: (1) of the pathways of net synthesis and depletion of citric acid cycle intermediates (flux) in muscle, and (2) the role of muscle in the catabolism of protein amino acids and in the provision of fuel to the liver as precursors of endogenously produced glucose in starvation and other conditions of negative nitrogen balance.